The present invention generally relates to magnetic storage systems. More particularly, the present invention relates to storage devices which employ magnetoresistive heads.
The trend in the data storage industry is to continually increase the data rate at which information is read from magnetic storage surfaces such as disk drives. Designers are developing high performance disk drives which tend to have magnetic domains positioned close together on the magnetic storage media, creating a high density of data recording. Additionally, these drives have greater rotation rates than lower performance drives, resulting in higher data rates. Higher data rates require faster voltage transitions at the input to the channel circuitry used to read data from the disk surface. Faster voltage transitions produce higher frequency components in the voltage input signal, thereby requiring a greater bandwidth from the channel circuitry.
Until recently, designers primarily used inductive heads to read data from these disk surfaces. Unfortunately, inductive heads are limited in the bandwidth at which they may function. Designers, accordingly, developed magnetoresistive (MR) heads which do not have the large inductance associated with previous heads. These MR heads, therefore, can accommodate higher data rates. Data is read from a disk surface by monitoring the changing resistivity of the MR head. The resistance of MR heads is a function of the strength of the magnetic field to which it is exposed. Since the resistance of the head varies with magnetic flux, the current through, or the voltage across the head is a function of the data written on the magnetic media.
This approach to reading information from the surface of a disk drive has proven to be very successful. However, as higher performance storage systems are developed, there is a continuing demand for greater bandwidth from the head and the read circuitry associated with the head. The ability to attain greater bandwidths is currently limited by the response of the MR head and the preamplifier used to generate read signals.
Several different preamplifier configurations are used in conjunction with MR heads. In general, these configurations differ in the manner by which they bias the head and in their approach to sensing changes in head resistance. A common configuration is a "current bias current sense" (CBCS) preamplifier, which biases the MR head with a constant current and senses changes in the resistance of the head by reading the signal variations in the current through the MR head. Other MR heads are implemented using current bias voltage sense (CBVS), voltage bias current sense (VBCS), and voltage bias voltage sense (VBVS) architectures to measure changes in head resistance. It is known that the CBCS configuration has a simple single pole response while the other configurations are more complex second order systems having multiple poles. These second order systems have slightly wider bandwidths than the single pole CBCS configuration. However, none of these existing architectures provide sufficient bandwidth to satisfy the ever-increasing demands of high performance storage systems. There is a need for a MR head and preamplifier implementation which has an increased bandwidth response.
A substantial portion of the bandwidth of signals produced by existing MR heads and preamplifiers is unusable because of pole frequencies created by factors such as the resistance of the head and the head lead and flex inductance. These pole frequencies impair the overall bandwidth of the preamplifier. One approach to recovering this lost bandwidth is to place several capacitors in the preamplifier to approximate the pole caused by the head resistance and the flex and head lead inductance. By using these capacitors, a general approximation of a zero can be generated which cancels the pole thereby extending the bandwidth of the preamplifier. This approach, however, is unsatisfactory for several reasons. First, because the capacitance selected is only an approximation of the value needed to cancel the pole, the pole is never completely cancelled. This can cause either a lower bandwidth or peaking in the frequency spectrum. Further, the addition of a zero to the preamplifier can result in feedback oscillation problems due to stray capacitance. These preamplifiers typically have high gains, making them susceptible to feedback oscillation. Another drawback of this approach is that it does not permit adaptive cancellation of poles. The poles of a typical MR head system vary as the magnetoresistivity of the head varies.
Accordingly, an approach to more accurately cancel pole frequencies is needed. Preferably, the approach should be implemented in a manner which permits adaptive (on a per-head basis) cancellation of pole frequencies to attain a wide bandwidth while maintaining a linear group delay. Little or no stray capacitance should be added to the preamplifier. Further, the approach should utilize existing read channel components to minimize or eliminate the need for additional circuitry.